fix(terminal-agent): tie agent lifetime to its owning browse server PID

The terminal agent is intentionally detached so it survives the
short-lived CLI launcher, but its real owner is the persistent browse
server. If that server crashed or was killed before running normal
shutdown, the agent was adopted by PID 1 and lived forever (#2019).

spawnTerminalAgent now requires an ownerPid and exports it to the agent
as BROWSE_OWNER_PID; all three spawn sites pass the server PID (cli.ts
cold-start, cli.ts supervisor respawn, server.ts watchdog). The agent
polls the owner with signal 0 every 15s (GSTACK_TERMINAL_OWNER_WATCHDOG_MS
to tune) on an unref'd timer and, when the owner disappears, exits
through the SAME cleanup path as an intentional SIGTERM shutdown — now
re-entrancy-guarded and also removing the terminal-internal-token file
alongside the port file and agent record.

Runtime test spawns a real agent tied to a throwaway owner process,
kills the owner, and asserts the agent exits and its discovery files
(terminal-agent-pid, terminal-port) are gone.

Reconciled with the watchdog commit's spawnTerminalAgent contract test
(process-liveness-windows.test.ts now passes ownerPid and pins the
BROWSE_OWNER_PID env forwarding).

Closes #2019.

Contributed by @csarigoz (PR #2530).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

browse/src/cli.ts

@@ -1130,6 +1130,7 @@ Refs:           After 'snapshot', use @e1, @e2... as selectors:
         const newPid = spawnTerminalAgent({
           stateFile: config.stateFile,
           serverPort: newState.port,
+          ownerPid: newState.pid,
           cwd: config.projectDir,
         });
         if (newPid) {
@@ -1222,6 +1223,7 @@ Refs:           After 'snapshot', use @e1, @e2... as selectors:
           spawnTerminalAgent({
             stateFile: config.stateFile,
             serverPort: respawned.port,
+            ownerPid: respawned.pid,
             cwd: config.projectDir,
           });
         } catch (err: any) {